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Kongens Lyngby, Denmark

Alam M.M.,Technical University of Denmark | Fabricius I.L.,Technical University of Denmark | Christensen H.F.,Danish Geotechnical Institute
Geophysics | Year: 2012

Deformation of a hydrocarbon reservoir can ideally be used to estimate the effective stress acting on it. The effective stress in the subsurface is the difference between the stress due to the weight of the sediment and a fraction (effective stress coefficient) of the pore pressure. The effective stress coefficient is thus relevant for studying reservoir deformation and for evaluating 4D seismic for the correct pore pressure prediction. The static effective stress coefficient n is estimated from mechanical tests and is highly relevant for effective stress prediction because it is directly related to mechanical strain in the elastic stress regime. The corresponding dynamic effective stress coefficient α is easy to estimate from density and velocity of acoustic (elastic) waves. We studied n and α of chalk from the reservoir zone of the Valhall field, North Sea, and found that n and α vary with differential stress (overburden stress-pore pressure). For Valhall reservoir chalk with 40% porosity, α ranges between 0.98 and 0.85 and decreases by 10% if the differential stress is increased by 25 MPa. In contrast, for chalk with 15% porosity from the same reservoir, α ranges between 0.85 and 0.70 and decreases by 5% due to a similar increase in differential stress. Our data indicate that α measured from sonic velocity data falls in the same range as for n, and that n is always below unity. Stress-dependent behavior of n is similar (decrease with increasing differential stress) to that of α during elastic deformation caused by pore pressure buildup, for example, during waterflooding. By contrast, during the increase in differential stress, as in the case of pore pressure depletion due to production, n increases with stress while α decreases. © 2012 Society of Exploration Geophysicists. Source

Koreta O.,Danish Geotechnical Institute | Myftaraga E.,POLIS University | Tanku E.,ALB STAR
Geotechnical Engineering for Infrastructure and Development - Proceedings of the XVI European Conference on Soil Mechanics and Geotechnical Engineering, ECSMGE 2015 | Year: 2015

In geotechnical engineering analysis, the uncertainties in the soil input parameters have always been a concern. Therefore, to take into account this issue, engineers are using reliability based design (RBD), which can deal with the probability of failure rather than only with the factor of safety. In this paper is presented a probabilistic analysis of a sheet pile wall penetrating clay, which has been analysed with the limit equilibrium method. It has been studied the impact of the variation of soil parameters (friction angle, cohesion) on the values of penetration depth, maximal bending moment, safety factor and probability of failure. In this structure, according to the failure mechanism, the soil is involved as load and also as resistance due to the active and passive earth pressures. © The authors and ICE Publishing: All rights reserved, 2015. Source

Alam M.M.,Technical University of Denmark | Hjuler M.L.,Danish Geotechnical Institute | Hjuler M.L.,Geological Survey of Denmark | Christensen H.F.,Danish Geotechnical Institute | Fabricius I.L.,Technical University of Denmark
Journal of Petroleum Science and Engineering | Year: 2014

Enhanced oil recovery by CO2 injection (CO2-EOR) is a tertiary oil recovery process which has a prospective for being used, at the same time, as an effective technique for carbon dioxide storage. There is a huge potential for additional oil production and CO2 storage in the North Sea depleted chalk reservoirs. North Sea chalk is characterized by high porosity but also high specific surface causing low permeability. A high porosity provides room for CO2 storage, while a high specific surface causes a high risk for chemical reaction and consequently for mechanical weakening. In order to address this issue we studied two types of chalk from South Arne field, North Sea: (1) Ekofisk Formation having >12% non-carbonate and (2) Tor Formation, which has less than 5% non-carbonate. We performed a series of laboratory experiments to reveal the changes in petrophysical and rock-mechanics properties due to the injection of CO2 at supercritical state. We analyzed these changes with respect to the differences in porosity, specific surface, pore stiffness, wettability, mineralogy and mechanical strength. We observed a 2-3% increase in porosity, a minor decrease of specific surface and consequently a small increase in permeability. A decrease in elastic stiffness is indicated by an increase of Biot's effective stress coefficient (α) by 1-2%. Nuclear Magnetic Resonance (NMR) data indicated no change in wettability and the samples remained water wet. We found that the effect of CO2 injection on both petrophysical and mechanical properties of chalk depends on carbonate content. Pure chalk with high carbonate content was found to be vulnerable to mechanical weakening due to CO2 injection, whereas, no significant mechanical effect was observed in the impure chalk of Ekofisk Formation. It should in this context be noted that the experiments spanned only 8 days, therefore long term effects cannot be ruled out. In spite of weakening of the chalk, we expect only minor mechanical effects, because the weakening also causes a lowering of effective stress due to an increase in effective stress coefficient. Extensive time-lapse monitoring strategies are required during a CO2-EOR process for the measurement of changes in reservoir properties that may cause deformation of and leakage from a reservoir. Results of this study will provide data for designing future monitoring strategies based on 4D seismic. © 2014 Elsevier B.V. Source

Alam M.M.,Technical University of Denmark | Hjuler M.L.,Danish Geotechnical Institute | Christensen H.F.,Danish Geotechnical Institute | Fabricius I.L.,Technical University of Denmark
74th European Association of Geoscientists and Engineers Conference and Exhibition 2012 Incorporating SPE EUROPEC 2012: Responsibly Securing Natural Resources | Year: 2012

Reservoir modeling and monitoring uses dynamic data for predicting and determining static changes. Dynamic data are achieved from the propagation velocity of elastic waves in rock while static data are obtained from the mechanical deformation. Reservoir simulation and monitoring are particularly important in enhanced oil recovery by CO2 injection (CO2-EOR) in chalk as, chalk reservoirs are vulnerable to compaction under changed stress and pore fluid. From South Arne field, North Sea, we used Ekofisk Formation chalk having approximately 20% non-carbonate and Tor Formation chalk having less than 5% non-carbonate. We studied difference in static and dynamic behavior. Furthermore, brine saturated data were compared with CO2 injected data to reveal the effect of supercritical CO2 injection in both static and dynamic elastic properties. We used strain gauges and LVDTs to measure static deformation. We observed lower dynamic elastic modulus for chalk with higher non-carbonate content at porosities lower than 30%. In 30% porosity chalk, dynamic compressional and bulk modulus were found significantly higher than the static modulus. Static measurements with LVDT were found lowest. The effect of CO2 injection was notable in dynamic elastic properties, while a possible change in static elastic properties was below detection limit. Source

Alam M.M.,Technical University of Denmark | Hjuler M.L.,Danish Geotechnical Institute | Christensen H.F.,Danish Geotechnical Institute | Fabricius I.L.,Technical University of Denmark
Proceedings - SPE Annual Technical Conference and Exhibition | Year: 2011

Changes in chalk due to EOR by injecting supercritical CO 2 (CO 2-EOR) can ideally be predicted by applying geophysical methods designed from laboratory-determined petrophysical and rock mechanics properties. A series of petrophysical and rock mechanics tests were performed on Ekofisk Formation and Tor Formation chalk of the South Arne field to reveal the changes in petrophysical and rock mechanics properties of chalk due to the injection of CO 2 at supercritical state. An increase in porosity and decrease in specific surface was observed due to injection of supercritical CO 2. This indicates that a reaction between CO 2 enriched water and particles takes place which smoothens the particle surface. Accordingly, partial increase in permeability was also noticed. An effect is also observed from the decrease in pore-space stiffness, calculated from sonic velocity. No significant effect on wettability as indicated by NMR T 2 relaxation time was observed. Rock mechanics testing indicates that in 30% porosity chalk from the South Arne field, injection of supercritical CO 2 has no significant effect on shear strength and compaction properties, while there is probably a slight decrease in stiffness properties. For both the Tor Formation and Ekofisk Formation, flooding with CO 2 after waterflooding does not seem to affect the shear strength parameters. For the Tor Formation, the elastic deformation parameters seem to be reduced after CO 2 injection. In contrast, the effect on the elastic moduli in Ekofisk Formation is insignificant. The time dependent properties for both Tor Formation and Ekofisk Formation do not seem to be affected by CO 2 flooding in the relevant stress regime. Generally, the change in both petrophysical and rock-mechanics properties is insignificant in Ekofisk Formation, compared to the changes in Tor Formation, most probably due to the very little contact cement in Ekofisk Formation chalk. Time-lapse monitoring strategies may be required during a CO 2-EOR process for the measurement of changes in reservoir properties that may cause deformation of and leakage from a reservoir. Results of this study will provide data for designing future monitoring strategies based on 4D seismic. Copyright 2011, Society of Petroleum Engineers. Source

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